Is it possible to mount my external HDD in Truecrypt under Whonix?
An easy way is to share the drive using Guest Additions, you could either share the encrypted drive and decrypt inside Whonix or directly share the decrypted (mounted) drive from host
https://www.whonix.org/wiki/VirtualBox_Guest_Additions#Shared_Folder
I think the latter is better, the vm won’t know about the password, but the contents will be decrypted anyway. Once you are done you could quickly dismount the drive from host without changing any settings in Whonix.
Oh, also you don’t have to share the whole drive. You could create a specific folder under the mounted drive to be shared with Whonix, or just share an existing one
Thanks so for. But what is about traces on the system? For example: if I open pictures, documents and or videos. Will informations about it saved on whonix? I wonder. If so: how can I fix that?
After importing the vm image, take a snapshot on VirtualBox and after any customizations which could include new software or simple updates, keep a ready-to-use snapshot and revert back after risky usage or each usage. Snapshots were actually recommended until recently on the Download page but Patrick says it (snapshot feature of VirtualBox) could cause data loss, it never happened to me so far, except once my disk was fulled up and the vm was unusable (never happened again). So backing up anything important and leaving plenty of free space on the drive the VirtualBox images are saved could save you from any data loss.
z answered most of this already.
For USB, see:
For other options, generally see:
Please see:
If so: how can I fix that?You don't want to know. ;) See also: https://www.whonix.org/wiki/FAQ#Is_there_a_substitute_for_Whonix.27s_lack_of_an_Amnesic_feature_.2F_Live_CD.2FDVD.3F_Forensics.3F No quick fix available as far as Whonix is concerned. If that is your priority, there is no alternative to Tails. Long term fix: join development or sponsor development of this feature. Hence, you don't want to know. :)
Just for the record, you could even run Whonix completely on RAM by using a ram disk software. For Windows, you could also set the path of TEMP or pagefile* or other things to a ram disk so to minimise traces further.
But it would require a high amount of RAM.
You could even create a ram disk first, then encrypt and mount it with Truecrypt and then install VirtualBox and Whonix inside this encrypted ram disk 
This way even the content on RAM disk would be encrypted
*Edit: On Windows, some (old) applications need a pagefile so I didn’t recommend to disable it. Set the pagefile to be on a ram disk, a small size (like <100 MB or 1 MB) would be enough, assuming you have a high RAM.
[quote=“z, post:6, topic:293”]Just for the record, you could even run Whonix completely on RAM by using a ram disk software. For Windows, you could also set the path of TEMP or pagefile* or other things to a ram disk so to minimise traces further.
But it would require a high amount of RAM.
You could even create a ram disk first, then encrypt and mount it with Truecrypt and then install VirtualBox and Whonix inside this encrypted ram disk 
This way even the content on RAM disk would be encrypted
*Edit: On Windows, some (old) applications need a pagefile so I didn’t recommend to disable it. Set the pagefile to be on a ram disk, a small size (like <100 MB or 1 MB) would be enough, assuming you have a high RAM.[/quote]
I am careful with claims to having substitutes for a real amnesic features, see: