Add documentation for zeroing Mac OS X guests before using the 'modifyhd --compact' operation

[Socratis]

In page 135 of the latest manual (4.2.4, as of this filing) it mentions the steps to zero out the free space for Windows guests (sdelete -z) and Linux guests (zerofree), but not for Mac OS X guests. An addition something like the following (in bold) would be helpful:

For this operation to be effective, it is required that free space in the guest system first be zeroed out using a suitable software tool. For Windows guests, you can use the sdelete tool provided by Microsoft. Execute sdelete -z in the guest to zero the free disk space before compressing the virtual disk image. For Linux, use the zerofree utility which supports ext2/ext3 filesystems. For Mac OS X guests, you can use the "Erase Free Space..." option in the built-in "Disk Utility" tool provided by Apple.

Not a technical writer, neither a native English speaker here, so feel free to adjust the wording at will ;)

[Frank Mehnert]

Thanks for the suggestion. A proper text will be included in the next maintenance release.

[Socratis]

Thanks a lot frank. Confirmed as fixed in 4.2.6.

[Socratis]

I hate to reopen something that has absolutely nothing to do with VirtualBox, but Apple (in their infinite wisdom) removed the DiskUtility functionality after OSX 10.11. The proper way to word this in the manual (so that it works in each and every version of an OSX guest) is to replace the existing:

"For Mac OS X guests, use the Erase Free Space feature of the built-in Disk Utility. Use Zero Out Data there."

with

"For Mac OS X guests, use the "diskutil secureErase freespace 0 /" command-line from an elevated Terminal."

[Frank Mehnert]

Thank you for the update! The next 5.0.x maintenance release will contain the change.

[Socratis]

Confirmed as changed in 5.0.14. Thank you...