`fastboot devices` command returns a device with question marks

Question

I am trying to root an Acer Iconia Tab A200 tablet using a script. I have adb working and fastboot works too, however, command fastboot devices returns question marks. E.g.:

List of devices attached
20819705215     device

$ fastboot devices
$ adb reboot-bootloader
$ fastboot devices
????????????    fastboot
$ _

^{Original screenshot of the terminal screen}

I execute the command fastboot devices in the bootloader which is already unlocked. Using Ubuntu 14.04, udev settings are already set (the file with vendors IDs) according to the ID result of the command lsusb:

SUBSYSTEM=="usb", ATTRS{idVendor}=="0502", ATTRS{idProduct}=="3201", MODE="0666", GROUP="plugdev", OWNER="ln"

What can I do to get the correct result? How can I make fastboot work well?

Answer 1

It can be a cable fault. Change the cable. In my Redmi 3 on a cable from an old Xperia P, I've had question marks. When I replaced the cable with a newer cable from Redmi Note 2 and voila - the correct number was displayed instead of the question marks.

Answer 2

Since you're on Linux, try sudo fastboot devices. I found that was needed with a later Linux than you have (RHEL7.9).

Answer 3

In my case, it was the phone, an unsupported/rogue Chinese firmware that had the option "fastboot" but was really just a nickname for a type of connection supported by their own software. Tried my setup (cable + PC) on different, 3, phones and it all worked except this one. Even the brand was murky with the only label on it being "k48".