This cure method is only for devices with proprietary Garmin operating system and doesn't work for devices with Linux-based OS. Garmin made some in the past and nuvi 5000 was one such. [Only registered and activated users can see links. ] which will give you a basic understanding of the problems 'curing' such devices as nuvi 5000 and 8xx models.

The most likely problem is that either the detail map as become corrupted or a faulty script generating the 'map needs update' message is causing it to stick in the boot cycle. If the problem was indeed faulty firmware the device would likely be defaulted to a permanent state of pre-boot. I'm inferring from your info that you need to manually enter preboot by holding the screen and powering on, correct?

Regarding the firmware, the Linux devices don't use gupdate.gcd, they use a file named something like "B07xxxx.update" in a similar way. They can also update from an *.exe file for older fw versions and can of course use an .rgn file in pre-boot. Regardless, I doubt your problem is firmware anyway and you likely need to find a way to remove the map or script causing the problem.

I own a nuvi 5000 so I'm happy to use it to investigate further on ways to solve your problem however I do think we need someone with Linux device software experience to make a 'cure firmware' so you can enter Mass Storage Mode.