Before buying a Time Machine drive: choose the backup target, not just its capacity

For macOS Tahoe 26, prepare the target for directly connected external Time Machine backups before choosing a drive. Record backup capacity, existing files and encryption needs. This source-only guide selects no product and excludes network targets and other macOS versions.

Prepare three consequential questions

1. How much space is intended for backups? Record the exact Mac model, installed macOS, internal storage capacity and proposed backup target. Apple advises at least twice the Mac's storage capacity: its example is a 2 TB backup disk for a 1 TB Mac. This is sizing guidance, not a guarantee of a particular backup history, available space or successful recovery.

Apple advises using the backup disk only for Time Machine. Its advanced-user exception permits separate APFS volumes for backups and other files; the backup volume should still be twice the Mac's capacity. Do not count the whole physical drive as dedicated backup space when another volume has a separate purpose. Apple — Connect a storage device and APFS-volume footnote.

An unknown Mac capacity or intended allocation leaves the sizing decision unresolved. Establish comparable units using the separate capacity guide [current article]; a nominal conversion alone establishes no usable backup space.

2. What existing data would setup put at risk? Identify the exact disk or volume and needed contents. Apple's Tahoe 26 guide says a Windows-formatted target cannot serve as-is; reformatting permanently removes data. If asked to erase, its alternatives are erase or another volume. First copy needed files to a different disk. The prompt is conditional; do not assume every candidate needs erasure.

The same guide warns that files kept on the backup disk are not backed up there and reduce space available to Time Machine. Keeping files beside a backup therefore does not establish that those files have their own backup. Apple — Choose a backup disk, steps 3–6 and closing warning, Tahoe 26.

If needed files or their separate preservation remain unverified, defer the change and consider another target. For ordinary Mac/Windows exchange, use the separate format-preparation guide [current article]; its Windows-compatible format advice does not establish Time Machine suitability.

3. Can the owner retain the backup password privately? Apple's Tahoe 26 password guide requires a backup password for encryption and for restoring files from encrypted backups. Plan its private retention before choosing encryption; never put it in public feedback. This guide supplies no forgotten-password workaround. Apple — About the Time Machine backup password.

Hand back the decision

Record the intended backup target, supported capacity/allocation, any unresolved connection or format requirement, existing-file preservation and password question. Keeping a suitable current drive or deferring purchase remains an option. Before comparing offers, separately establish the exact model, accessories, seller, condition, budget and availability. These notes establish neither purchase fit nor that a backup exists or can be restored.

Evidence: Three Apple pages checked 2026-10-05T09:04:29Z; the general guide is dated July 6, 2026, while the two guides are pinned to Tahoe 26 without a displayed update date. The author completed sixteen synthetic manual decision checks. A distinct same-operator checker reopened the three Apple sources through 2026-10-05T09:15:26Z and rechecked those decisions plus six additional boundaries at 2026-10-05T09:16:51.434437Z. No drive, file copy, erase, encryption, backup, restore or purchase was performed. Same-controller checks provide no independent formal review. Recheck before changed software, target or source wording, and by 2026-11-05T00:00:00Z. Feedback should name the disputed public step without file details or passwords. This follows the purchase standard [current article].