What if you connect your usb hard drive to a pc, where M: isn't available any more?

@echo off

rem Set virtual drive letter to be used by foobar ; should always be free, at least from mounted & free drives
set _newdrive=Y:
rem Set relative path to foobar executable from the root of your USB drive
set _fb2krelpath=Install\fb2k\foobar2000.exe

:start
set _curdrive=%cd:~0,3%

if exist %_newdrive% ( 
	rem if newdrive letter already used for a virtual drive we can remove it, use it and restore afterwards.
	for /f "usebackq tokens=1,2 delims==> " %%a IN (`subst`) DO if %%~da==%_newdrive% set _olddrive=%%b
	
	subst /d %_newdrive%
	
	if exist %_newdrive% ( 
		echo "%_newdrive% deja utilise !"
		pause
		goto end
	)
)

subst %_newdrive% %_curdrive%

%_newdrive%/%_fb2krelpath%

subst /d %_newdrive%
if exist %_olddrive% subst %_newdrive% %_olddrive%

:end
set _curdrive=
set _olddrive=
set _newdrive=
set _fb2krelpath=

exit

@echo off

rem Set virtual drive letter to be used by foobar ; should always be free, at least from mounted & free drives
set _newdrive=Y:
rem Set relative path to foobar executable from the root of your USB drive
set _fb2krelpath=Install\fb2k\foobar2000.exe

:start
set _curdrive=%cd:~0,3%

if exist %_newdrive% ( 
	subst /d %_newdrive%
	
	if exist %_newdrive% ( 
		echo "%_newdrive% already in use !"
		pause
		goto end
	)
)

subst %_newdrive% %_curdrive%

start "foo" /b %_newdrive%/%_fb2krelpath%

:end
set _curdrive=
set _olddrive=
set _newdrive=
set _fb2krelpath=

would be nice if support for relative paths was part of foobar itself though. not just for music but for custom button images etc.

Everytime you attach your drive it will be mounted to this path.

You copy the last line with dashes, didn't you?
Try harder

PS put the script on your USB and run it

' TARGETDRIVE-- This is an open drive letter where your USB drive can be persistently mounted on
' each computer on which you will use this script. In Foobar you will point your
' Library to this drive to set the absolute paths to your media.
' SUBDIR-- Your probably won't choose to change this value, but it would allow you to map a
' subdirectory of your USB drive as the root of TARGETDRIVE.
' FOOBARRELATIVEPATH-- This string is the relative path from the working directory of this script
' to the foobar executable. If you put this script in the directory containing the
' foobar folder, this should work fine.
'************************************************************

Const TARGETDRIVE = "U:"
Const SUBDIR = "\"
Const FOOBARRELATIVEPATH="foobar2000\foobar2000.exe"

'************************************************************
Dim WshShell, FSO, sCmd, sCurrDrive, iResult
Set WshShell= Wscript.Createobject("Wscript.Shell")
Set FSO= Wscript.Createobject("Scripting.FileSystemObject")

sCurrDrive = FSO.GetDriveName(Wscript.ScriptFullName)

'delete substituted drives on the target drive letter-- won't affect mounted or real drives.
WshShell.Run "%comspec% /c if exist " & TARGETDRIVE & " subst " & TARGETDRIVE & " /d", 0, True

'Substitute USB drive/directory as target drive
sCmd = "%comspec% /c subst " & TARGETDRIVE & " """ & sCurrDrive & SUBDIR & """"
iResult = WshShell.Run(sCmd, 0, True)

If iResult > 0 Then
iResult = MsgBox("Error creating substitute drive " & TARGETDRIVE & ", launch Foobar?", vbYesNo + vbInformation, "Drive Substitution")
If iResult = vbNo Then Wscript.Quit
End If

' Run Foobar, and then delete subst drive letter when it exits
' (the two are done as one command line, so that Wscript can exit leaving behind only a
' hidden Cmd window to delete the drive letter when the program exits- much less memory.)
sCmd = "%comspec% /c """ & FOOBARRELATIVEPATH & """&&subst " & TARGETDRIVE & " /d"
WshShell.Run sCmd, 0, False

There will be no need for that with next big foobar release.

On what authority is this rumour?