Deleting at ND2901 or computer usually just delete the folder entry.
So there are high probability you can recover your data if you had not write any data to ND2901 after delete something.
If that is the case, you can recover files using data recovery software running at PC.
Pls check here for more detail.
https://www.lifewire.com/free-data-recovery-software-tools-2622893
