Linux /usr/bin file disappears after reboot

"I'd agree with CarlF's diagnosis -- likely the system is running in a RAMFS, loading from ROM every boot. To permanently change it you'd need to reflash the ROM.

There might be other options, but you'll have to give us particulars -- what model Thecus are you using?

I found a page on running Debian on a Thecus n4100. It indicates the n4100 has a MiniPCI slot that could be used for adding a CF card to provide some writable storage, or an alternative boot partition. Your model might have a similar feature.

Alternatively, you may be able to mount a network share (CIFS/SMB/NFS) and load it with your software."