As the topic title suggests, will Thailand post allow to send a USB drive in an envelope abroad by registered mail, or should I better use SD card?

Why wouldnt they? And if you are concerned why not just use your other suggested option? 

Why not just use Dropbox?  And encrypt it if you're worried about security...though obviously, if you're going to send it through the mail, you're not worried one bit about that.

18 minutes ago, Chou Anou said:

Why not just use Dropbox?  And encrypt it if you're worried about security...though obviously, if you're going to send it through the mail, you're not worried one bit about that.

 

You can place encrypted files on a USB drive.

There is absolutely zero reason that Thailand Post would deny sending a USB or SD card.

 

BUT

 

Just how much data are you sending? DropBox and the like are great and fast (instant) for relatively small amounts, but when sending many terabytes posting media (even hard drives) is still faster.

USB Flash drive, fine, but when I send such, I use an SD card sandwiched between a couple of semi stiff cards.  Same result at the recipient's end but flatter and less noticeable enroute.

Sending the files over the cloud is no option, because the recipient is of the dumb category , who in almost 2 weeks hasn't managed to download the RAR files from my Google drive without extracting them or even adding OSX files to the RAR file. Forgive him he's an Apple user 

 

Reason I asked is that if you send documents which are attached by a staple, and they feel or discover the staple, they refuse to send or use a different more expensive shipping method.

 

Been there, got the T-shirt.

 

So since a USB drive has metal inside, I thought the same rules would apply.

Have sent microSD chips inside Thailand and to USA. Put several printed magazine pages nicely folded and tape the non-gold side with a small piece of usual Scotch tape.