hey, i have a low storage server and would like to use dynmap. Is there a possibility to run dynmap on a USB / disk?

no.

is there any other way to run it

Dynmap needs to be in the "plugins" folder, if it's taking too much place, you can try to lower the resolution, remove some maps (don't forget to remove the files), and I'm not sure if that works but set the storage system to SQLite. If you have MySQL, then I think this will be helpful for you.

okay thanks I'm going to inform myself about it thnx

I don't know whether there would be any issues moving the dynmap plugin (or simply the tiles) to an external location with a symbolic link, but I'd definitely not be recommending doing so.

Change from png to jpg in the configuration. Small loss of details but massively less storage required

Change from png to jpg in the configuration. Small loss of details but massively less storage required

Just to note that this is now the default option in the config.