WARNING !!! Unrecoverable bricks signalled on 3.71 M33 !!!

From the sounds of it, it is now possible to cause a FULL UNRECOVERABLE brick on your 3.71 M33-2 PSP.

Symptoms:
You used USB flash access to write something to flash0/1/2/3, either in recovery or in VSH after re-routing USB. Although Dark_AleX fixed the bug that was present in 3.60 M33 and 3.71 M33-1 (which caused files larger than approx. 60kb to be corrupted), flash writing is still unreliable. It seems the nand partitioning is messed up after a USB flash write.

After flash access through USB, you either get an infinite BSOD or a PSP that doesn't boot at all. The PSP can't be recovered by Pandora or any of the Cemetery sticks!

Precautions:
Don't use USB flash write access when on 3.71 M33-2 until further notice! If you did and you get a BSOD, don't press O! It will cause an infinite BSOD loop that can't be fixed. According to some reports in other threads, when you hard power off your PSP at the BSOD (power switch or battery out) you have a small chance to get the PSP booting again, but some things will not function since flash1 is not correctly initialized. Probably a "restore default settings" may fix this, otherwise, try the flash1 format from recovery.

Solutions:
When you have a working full nand dump safely stored somewhere, use the Cemetery V3 stick to restore it to your PSP. On the fat, you can also use Cory's upNandTool or a similar tool, on the slim you can also use Cemetery V1 or V2. Don't try to use Pandora, because it does a logical restore and for this problem, you need a physical restore. Remember that it's impossible to restore any other PSP's nand dump, because you will screw up IDStorage beyond any repair!!!

When you don't have a nand backup but your PSP is a fat, get a nand backup from another PSP. It may be of any fw version, although only 1.50 will boot instantly. The important part is the partitioning, not the nand contents. Before doing anything to your bricked PSP, use Pandora or any Cemetery stick to create a full backup of your nand! Store it safely on your PC, and maybe a copy on another media. After making sure your nand backup is safe, use Cemetery V3 to restore the other nand dump. Pandora will not work because you need a physical restore, not a logical. When you restored a 1.50 dump, your PSP should now boot again. With any other firmware, it will still be a brick. You should now be able to use Pandora or Cemetery to unbrick it. All that remains is to restore your own IDStorage, or your PSP will be homebrew only. Use the dump you created from your own bricked PSP and Cory's upNandTool to restore IDStorage only without overwriting the other nand contents. Now your PSP should be fully functional again.

When you don't have a nand backup and your PSP is a slim, you're screwed. Restoring another PSP's nand dump won't work because of the protections Sony added to the slim on nand level, including signing and encryption. If you still want to give it a go, you can try the procedure described above for the fat. But use the 64M version of upNandTool instead. And only use another slim's nand dump, a fat's dump will definitely not work.

FINAL WARNING !!!
Especially on the Slim, before doing anything that writes to your nand (including regular updates), create a nand dump! Store it on your PC and on another media and make sure it's safe! Currently it's your only chance of unbricking a full brick!

[EDIT] It seems to me that Sony is no longer afraid of bricking their customer's PSPs. My guess is that part of the nand protection they already put in the slim's hardware, has been incorporated into the 3.71 firmware, causing all kinds of corruptions whenever a flash write is done. Although it still could be a stupid bug...