I brought my MacBook Pro 1 year ago, at that time it runs OS X 10.9 . At that time I connected an external 2TB HDD (HFS+ single partition, not connected all the time) to it via USB as a Time Machine backup destination. Later I turned off Time Machine and use the HDD to store some pictures. Months after that I upgraded to OS X 10.10 and now 10.11 Beta 1.
Today I noticed my disk space not enough so I scanned the disk for big files, only to find there is a
lost+found folder in my external HDD, which takes nearly 1TB of space. The content of which is files I deleted during the days, most of them originated from internal SSD (And I never put them into that HDD), the create time of which are all in 2014. All the files are named like
iNodexxxxxxxxxx and directories
Why are my deleted files now found in an external disk? Is it safe to delete the whole folder without affecting anything outside?