So we must know more:
  • Is your 2599 the C4 device HWID 1947 or the other HWID 1758?
  • Which Drive model's RGN file did you flash? Please give that firmware's HWID or preferably a link to the actual firmware you flashed.
  • You said when trying to re-flash that you get a message "USB is not available" which I guess is from Updater.exe. Please state exactly what that message was or preferably post a screenshot of it.
  • Also what exact name did you give to the RGN file which you were attempting to re-flash when you got that message? e.g.: <any_name>.rgn or this convention XXXX01000xxx.rgn where XXXX is a 4-numeral HWID and xxx is a 3-numeral firmware version.
  • It probably can't be seen by a PC in Mass Storage Mode because it's locked into pre-boot mode now. However, when connected to PC by USB is it recognized in Windows' Device Manager under 'Garmin Devices' list?

[Only registered and activated users can see links. ]

Advise of anything else at all you feel might be relevant too. The more information you give the better and at very least answering all of the above questions is essential. Depending on (1) what you've done, (2) how the device is presently acting and (3) the results of some further testing it may be anything from an easy fix to a hard-bricking that's impossible to repair. However while the device is alive and has pre-boot available there's definitely hope of restoring it.