The Backup That Did Not Fit
A COPA batch is preparing a backup practical. The data to protect is 28 GB of project files (documents, images and short videos) held on the file server. The target is a 64 GB pen drive that Windows reports as 59.6 GB, of which 41 GB already holds last month's batch videos that the instructor wants to keep for a fortnight. Compression is available with 7-Zip, and last month's similar data compressed at a measured ratio of 1.3:1. After the full backup, 7 daily incremental backups of 400 MB each are to be stored on the same drive. Measured write speed to the pen drive is 22 MB/s and verification read-back is 30 MB/s. The whole job, including verification, has to complete inside a 45-minute practical slot, and the trainee's plan is: compress, copy, verify, then add the seven increments.
1. Carry out the space calculation in consistent units and state whether the plan fits. Show every step.
Read the worked answer
Work in one unit - megabytes, 1 GB = 1024 MB.
- Data: 28 × 1024 = 28,672 MB
- Compressed: 28,672 ÷ 1.3 = 22,055 MB ≈ 21.5 GB
- Increments: 7 × 400 = 2,800 MB ≈ 2.7 GB
- Total required: 22,055 + 2,800 = 24,855 MB ≈ 24.3 GB
- Free space on the drive: 59.6 − 41 = 18.6 GB ≈ 19,046 MB
- Shortfall: 24,855 − 19,046 = 5,809 MB ≈ 5.7 GB
The plan does not fit. Note the trap: the headline '64 GB drive, 28 GB of data' looks comfortable, and the arithmetic only fails once the 41 GB already occupied is subtracted. Always compute free space, never nominal capacity.
Time check, as a second constraint: write 28,672 ÷ 22 = 1,303 s ≈ 21.7 min; verify 22,055 ÷ 30 = 735 s ≈ 12.3 min; increments 2,800 ÷ 22 = 127 s ≈ 2.1 min. Total ≈ 36 min of pure transfer, leaving about 9 minutes for compression time, which was not measured. Time is tight but feasible; space is the binding constraint.
2. Propose three workable options with their trade-offs, and recommend one for the 45-minute slot.
Read the worked answer
- Move last month's 41 GB of videos to the server or a second drive first. Frees the whole problem: 24.3 GB fits into 59.6 GB with roughly 35 GB spare. Trade-off: needs a destination with 41 GB free and about 32 minutes at 22 MB/s, which consumes the practical slot. Best done the previous day.
- Second pen drive / external HDD for the increments. Full backup (21.5 GB) goes on the existing drive; daily increments (2.7 GB total) go elsewhere. Trade-off: two media to track, and the increment chain must be restored together with the full - a classic restore failure if the pairing is not labelled and documented.
- Exclude the short videos and back up only documents and images. Trade-off: fastest, but silently drops exactly the data most painful to recreate, and it teaches trainees a bad habit - selective backup without a documented, approved reason.
Recommendation for the slot: if the videos cannot be relocated today, split the media (option 2) and use the slot to teach both halves of the discipline - the full backup plus a verified restore of one folder, not the transfer of everything. A backup that has never been test-restored is an assumption, not a backup, and the restore drill fits far better inside 45 minutes than a capacity fight.
3. A trainee asks why a 64 GB pen drive shows 59.6 GB. Answer the question and explain why the confusion matters in this specific calculation.
Read the worked answer
Drive manufacturers quote decimal capacity: 64 GB = 64,000,000,000 bytes. Windows divides by 1024 at each step (1024³ bytes per GiB), so the same drive shows about 59.6 GiB. Neither figure is dishonest; they are different units sharing a similar name, which is why the standards now use GB for decimal and GiB for binary.
It matters here because mixed units cause errors in both directions. If the 28 GB of data is measured in GiB (as Windows reports it) but the free space is computed from a decimal 64, the free-space figure is overstated before the 41 GB is even subtracted. In this case the unit gap (~4.4 GB) is smaller than the shortfall (~5.7 GB), so the conclusion does not change - but a trainee who lands just inside the limit with mixed units may well be over it. The rule to enforce: convert everything to one unit first, write the unit beside every number, and treat formatted capacity minus existing data as the only usable figure.