Most sdcard ports are unbootable unless you use something like Plop to boot them. Using a Pi maybe tempting, but not all programs run on ARM processors. Another to avoid is Bay Trail and Cherry trail systems, Linux support for them is atrocious. Nothing is really write protected unless it has a hardware switch, but you need enterprise level hardware for it.
Oh, one thing before going further, flash memory has limited writes (obviously), but sd cards usually have fewer writes than thumbdrives. However, many thumb drives have cheap voltage regulators which cause them to die off before even a micro sd would have become worn out. Any small flash memory system should be considered disposable (which is why they should not be your sole backup system).
That said, there are several options, going from easy to difficult...
You can get thumbsticks that only hang out 1/4in., this would let you swap systems at will. There are even micro sd adapters that stick out no further.
You can install a thumbdrive image straight to a normal hard drive, leaving it read only. That's all these thumbsticks are doing is acting as a hard drive with a system set to read only. Use Universal USB Installer, enable "view all drives" and point it at the drive you want to use, it should do the rest (note I have not tried this, but it should work). Of course this means wasting a hard drive, and carrying that extra weight, other methods listed here allow you to remove all other drives. This is probably the best option... I recommend finding an older Intel 30GB ssd though. You might think the usb would be more efficient power wise, but USB is not as optimized as a nice ssd, and you can't be sure hwo a system will react to standby mode when running off a usb stick. This is something you may want to test before you go any further.
There is an old Hp netbook (Hp Mini 1000) that had a recessed USB port. It was meant for a proprietary HP accessory, the Mini Mobile Drive, but was actually just a recessed USB port with a proprietary slot shape. While you could just go buy a Mini Mobile drive (I found none on ebay,
Amazon wanted $90 for a 4gb version...), it's really easy to open it up, cut a small chunk of plastic and plug in a small usb drive in it (I put a bluetooth adapter in mine). Bonus, this has the best keyboard of any small computer you will find, had higher resolution than most at the time, 3 different screen sizes (9,10 and 11.6) and came in artsy versions. Of course, as a netbook, it will be a bit slow (it supports 2gb ram), but for $75-100 (
Ebay), it's not going to be heartbreaking if it gets trashed.
Guide to hacking the port, this guy relocated his to be a normal USB, but of course you don't need to, it just shows what it is and where. While slow and not fantastic battery life, these where great little netbooks, they could even run OSX with minimal effort (chrome should also work). They are pretty durable as well, if you want disposable, this is one of your best options. Being a netbook, Linux support should be great. While you may scoff at a netbook, considering you will be running Linux off a USB stick, the netbook processor is probably not going to be that big of a bottleneck.
Find a laptop with an Express Port (for small, look at the Lenovo S10E, has to be the E variant), then get an express card with a single usb port. Open the card and hack the insides of it, it should be just a pass through as the express port is just a mini pci-e port, which contains a sata connection and usb connection, Before anyone goes and gets any ideas, many Express Ports have the sata lines removed for cost cutting.
Speaking of mini pci-e...
Find a laptop with dual internal mini-pci-e slots and use the second one (as above, the sata lines are probably cut, but soem companies do hook them up, research!). You can get a mini-pice-e to usb adapter but odds of it fitting inside with the cover in place is slim. The other method is to solder in a usb port and place it where the HD or CD would normally sit. The Lenovo T410s (must be an S model) is a bit ugly, heavy, and doesn't have great battery life, but they are relatively cheap ($100-150 I think), plentiful, available with a gorgeous 1440x900 screen, and Nvidia graphics. With a (first gen) I5 or I7 and 8gigs of DDR3 ram, they can be a real powerhouse. Also, Lenovo Linux support is pretty much second to none, this is one of the few laptops where Linux doesn't significantly alter battery life and Mac can probably be run as well.
A few notebooks have a usb port on a secondary daughter board, which can be unbolted and stashed elsewhere (hd or cd bay) inside the laptop to create a flush look. While an easy mod and sounds simple enough, the trouble is finding a compatible laptop. The problem isn't just one with a daughter board (not that common), but one with a daughter board and a long enough cable to relocate it. The Sony SZ series (13in screen) is the only one off the top of my head I can think of capable of it. These laptops are very nice, and while you can find them relatively
cheap, certain parts and repairs on them are quite expensive and difficult when they do break. These were light weight, high end, exotic systems and sold to niche users, so they are often are in great shape (they have very nice keyboards, touchpads, and screens too). The later models 550 and up (EU) and 650 and up (US) suffer from the Nvidia x400 soldering problems, but since they have dual graphics cards, you can still use the internal Intel video system if it fails, but most still fully operate since it was only used part time. The later models also had some pretty high end C2D processors that could hang with early Core I5 processors when given enough ram (these support 8gigs DDR2), but that much DDR2 is not cheap (comparatively speaking), not that Linux needs it anyhow. Some versions of Ubuntu do not like to suspend on these models, but that is an easy fix (use dconf-editor ). This too could run Mac in a pinch, at least the later versions.
Tip...
On any of these laptops, look for ones labeled "for parts", many times companies will only remove the HD. I've gotten systems dirt cheap this way ($40 Lenovos). Make sure it boots to bios, is not bios locked (Lenovo!) and that they include the drive caddy and cover or be prepared to buy them. Buy from someone with a good rep, assume the battery is toast. I only buy if I see the laptop I am buying in the photos, not a stock photo, and that it shows it has booted to bios. I also look them over well in the photos. One companies grade B is another companies grade D, and a small scratch on the center of the screen is different than a large scratch on the palmrest, so you want to know what you are getting not some computer illiterate lackey's assessment.
